A transfer-join bonding technique is a combination of a mechanical lock-and-key and metal/adhesive hybrid three-dimensional interconnect that is suitable for fine pitch chip-chip, chip-wafer and wafer-wafer connections. An important feature of this process, is that the lock-and-key structure safeguards the wafer, chip or wafer/chip pair from lateral shifts during lamination (i.e., bonding). Pitch is the distance between two lock-and-key interconnect structures.
Transfer-join bonding relies on local pressure at the transfer-join bonding interface. As such, transfer-join bonding is sensitive to bonding area/pitch. A smaller bonding area usually results in bad bonding quality. For example, for lock-and-key interconnects of a given size, placing the interconnects at a smaller pitch can cause a bond to fail, whereas the same interconnects placed at a greater pitch will bond. In other words, bonding is dependent on layout and density, because local bonding pressure is proportional to an equivalent area occupied by each interconnect. Smaller pitch means smaller equivalent area which means less local bonding pressure and worse bonding quality.
Accordingly, for a smaller bond area, a larger pressure needs to be applied to the wafer, chip or wafer/chip pair in order to form a good bonding interface. A higher bonding force can be used to improve the bonding quality. Due to tooling limitations and/or the process pressure impact to the device, there is a maximum limit at which pressure can be applied to the transfer-join bonding. As a result, this maximum limit on pressure effectively sets a lower limit for the smallest bonding area that can be implemented. At the same time, with the scale-down of bonding print area, it is desirable to keep as large a bonding area as possible to make a good bonding interface and to minimize bonding interface resistance.
Therefore, transfer-join bonding techniques that are scalable and that maximize the bonding area would be desirable.